1. Field of the Invention
The present invention relates to a preheating mechanism for use with a duplex electric-arc furnace system which performs alternate preheating and melting operations in two electric-arc furnace units for the purpose of improving the efficiency of melting in the overall system and for reducing the power consumption through preheating operations. The present invention also relates to a raw material-preheating apparatus that reduces the power consumption of melting operations by preheating with a fuel the metal scrap charged into the electric-arc furnace as a raw material.
2. Prior Art
(1) A conventional duplex electric-arc furnace system which performs alternate preheating and melting operations in two electric-arc furnaces is shown in FIG. 9. The preheating furnace in this system is equipped with a cover that can be swung to avoid any interference with the movement of the electrodes. This design layout is disadvantageous in that a considerable amount of space is required to permit unimpeded movement of the furnace cover and that the swinging of said cover involves an undue long cycle time.
FIG. 9 shows the case where a set of electrodes 2 are placed in a furnace shell 1a for performing the melting operation and a preheater 3 is placed in the other furnace shell 1b for carrying out the preheating operation. In order to perform the preheating operation in the shell 1a and the melting operation in the shell 1b, the cover 4 is swung about a king pin 5 along the path indicated by the two-short-and-one-long dashed line A. The post 6 about which the preheater 3 is swung must be located at a position that clears the circular area defined by the line A having the radius r. With a furnace of twenty tons capacity, the value of r is typically about 8 m. If the post 6 is so positioned as to clear the circle having this value of radius, the preheater will require a very large area measuring a length (L) of 17 m and a width (W) of 16 m. In addition, the preheater 3 must be swung through an angle of about 270.degree., which requires a prolonged time to cover. Furthermore, in order to avoid heat radiation from the preheater 3 and the hazard of dropping materials, the operator must remain outside of the above specified area of the preheater as long as the operation of shifting the preheater from the shell 1b to the shell 1a is continuing.
(2) In order to reduce the power consumption of the melting operation in an electric-arc furnace, the scrap charged into the furnace is preheated by burning a fuel in a combustion chamber mounted on top of a vertical hole bored through the scrap (see, for example, Laid-Open Japanese Utility Model Publication No. 142698/1984). The present inventors previously filed Japanese Patent Application No. 132895/1984 (filed on June 29, 1984 and not yet laid open) for an improved version of this method, wherein the exhaust gas is recycled for the purpose of reducing the required amount of preheating fuel and minimizing the oxidative loss of the scrap. The success of this scrap preheating method depends on whether the following three requirements are met: (1) combustion of the fuel and gas is completed within the small space of the combustor; (2) the combustion gas of the fuel and gas is discharged at a pressure high enough to overcome the pressure loss that develops as the combustion gas permeates through the scrap layer; and (3) the gas-recycling system experiences a small pressure drop.
One problem associated with this preheating method is instability of the gas combustion resulting from direct mixing of the burning flame with the recycled exhaust gas of a low oxygen content. This leaves soot as the unburnt component, which is deposited on the scrap, hood, ducts and dampers. The soot deposit will deteriorate the environment of the shop unless some provisions are made such as, for example, periodical soot removal. In order to avoid direct mixing of the burning flame with the recycled exhaust gas, the latter must be supplied at high speed, but then, the exhaust gas recycling system will experience an increased pressure drop. If the head of the blower is increased so as to compensate for the pressure drop, the power saving accomplished by the preheating operation is partly cancelled by the increased power consumption of the blower.